LibGUI: Move common action definitions to CommonActions.cpp

This makes Action.cpp itself only talk about the Action object, and
makes it easier to navigate.

Userland/Libraries/LibGUI/CMakeLists.txt

@@ -22,6 +22,7 @@ set(SOURCES
     ColorInput.cpp
     ColorPicker.cpp
     ColumnsView.cpp
+    CommonActions.cpp
     CommonLocationsProvider.cpp
     ComboBox.cpp
     Command.cpp